In wavelength division multiplexing (WDM) optical communication system, polarization scrambling may be applied to an optical signal in order to suppress the occurrence of polarization dependent gain (or polarization dependent loss) held by an optical repeater, and polarization hole burning (PHB), or to alleviate quality deterioration of a signal waveform due to nonlinearity of an optical fiber (for example, refer to Japanese Laid-Open Patent Publication No. 9-149006).
Moreover, in WDM optical communication system, an optical signal of a modulation format different from an existing channel may be added as a new channel at the time of upgrading the system. In this case, if wavelengths of optical signals of different modulation system are arranged adjacent to each other, performance of the system may be deteriorated resulting from a difference in the cross-phase modulation effect due to the difference of the modulation formats. In order to avoid this, a technique has been proposed where wavelength bands in which each optical signal is arranged, are grouped for each modulation format, and a guard band is provided between respective wavelength bands (for example, refer to Japanese Laid-Open Patent Publication No. 2006-50640).
To realize a large capacity of the optical communication system, research and development of a digital coherent receiver has been currently in progress. In the digital coherent receiver, polarization information included in received light needs to be restructured, following polarization fluctuations in the optical signal, which may occur at a certain rate due to an environmental change, for example, in an optical transmission path. This restructuring of polarization information of the received light is performed by subjecting a photoelectrically converted reception signal to digital signal processing by using a finite impulse response (FIR) filter or the like. In restructuring of polarization information by means of digital signal processing, the arithmetic processing thereof is under a high load, and hence there is a limitation on high speed processing. Therefore, a configuration in which the above-described polarization scrambling is basically not applied to an optical signal received by the digital coherent receiver has been studied.
When the existing optical communication system applied with polarization scrambling is to be upgraded by using the digital coherent receiver, a polarization scrambled optical signal and a non-polarization scrambled optical signal are simultaneously transmitted inside the optical fiber. At this time, the polarization state, phase, and amplitude of the non-polarization scrambled optical signal may fluctuate timewise due to the cross-phase modulation effect and Raman amplification effect resulting from the nonlinearity of the optical fiber. If the polarization state of the optical signal received by the digital coherent receiver fluctuates at a speed faster than a normally assumed speed due to an influence of polarization scrambling applied to other optical signals, digital signal processing for restructuring the polarization information cannot follow the fast fluctuation of the polarization state. Therefore, there is a problem in that quality deterioration of the signal waveform occurs at the time of restructuring the polarization information of the received light.
The above problem is not limited to the case in which the optical signal is received by using the digital coherent receiver, but is common to various types of reception systems using the polarization information of the optical signal. Moreover, the polarization state of the transmitted light may fluctuate faster than the normally assumed speed resulting from an interaction between respective optical signals, due to some sort of phenomenon other than polarization scrambling. Therefore, in reception processing of the optical signal using the polarization information, it is an important issue to reduce the influence of polarization fluctuations faster than the normally assumed speed, on the reception processing.